In optical fiber communication systems and optical fiber measurement systems, processing is required in variety on the intensity, phase, polarization and so forth of light waves. For this purpose, light intensity modulators, photonic switches (optical switches) and optical attenuators, for example, have been developed and put into practical use.
An optical waveguide technique is available as one of such optical processing techniques. The optical waveguide technique has advantages that waveguides can be of low voltage, can be integrated with ease, can be strong and can be formed in large numbers and at a low cost. What are used as substrate materials for optical waveguides are rich in variety, such as semiconductor materials, oxide crystal materials and glass materials. In particular, in optical waveguide type intensity modulators, oxide crystal materials having electro-optic effects as typified by ferrodielectric lithium niobate and lead lanthanum titanate-zirconate are widely used as substrate materials.
Here, how control electrodes operate which are provided in contact with or proximity to optical waveguides is explained with reference to the following mathematical expressions (1) and (2):Δφ=(2Π/λ)·δn·L  (1),δn=(−½)·n3·r·Γ·(V/G)  (2).
In the mathematical expression (1), Δφ is the level of a phase change produced by applied voltage, λ is the wavelength of an input, δn is the change in refractive index that is to be induced, and L is the electrode length.
In the mathematical expression (2), n is the refractive index, r is the electro-optical constant of a substrate crystal, Γ is the parameter of light wave-electric field overlap, V is the applied voltage, and G is the gap between electrodes.
As can be seen from the mathematical expressions (1) and (2), in order to attain the phase change Δφ at a level as high as possible, it is necessary to attain the refractive index change δn at a value as large as possible.
For this end, the following points are important.
(1) The gap G between electrodes is set small as far as possible. This can make the electric field have a high intensity.
(2) The parameter Γ of light wave-electric field overlap is set large. For this end, the electrode width is set as large as possible to make lines of electric force as many as possible in a deep region in the substrate material in respect of optical guide mode distribution so that a sufficiently strong electric field can be applied.
In general, the electrode width is set maximum within the area limit of optical waveguides used, and hence the freedom of studies is limited. Accordingly, it is studied how the gap G between electrodes is made narrower.
Here are beforehand properly shown a typical structure that is common to waveguide type optical control elements making use of a lithium niobate substrate and utilizing the electro-optic effect of such a substrate material, and how to make this structure. As an example of such waveguide type optical control elements, the constitution of an optical circuit of a Mach-Zehnder interferometer type light intensity modulator and its constituent factors are exemplified in FIG. 6.
Ti metallic stripes of about 10 μm in width each are formed on a lithium niobate substrate 11 by photolithography. Thereafter, the substrate with stripes is processed at about 1,000° C. to make Ti atoms diffuse into the lithium niobate substrate. As the result, the Ti atoms diffuse thereinto in its areas of a depth of about 5 μm and a width of about 10 μm each to produce a distribution. At this part, the refractive index comes high substantially in proportion to the concentration of Ti atoms. Regions where this refractive index has come high serve as optical waveguides 12a, 12b, 13a, 13b and 15. In these regions, light waves of from 1.3 to 1.55 μm in wavelength which are used in optical fiber communication systems are propagated in a single mode.
Subsequent to the formation of the optical waveguides, SiO2 is vacuum-deposited on the surface of the lithium niobate substrate in which the optical waveguides have been formed, to provide a buffer layer thereon. The buffer layer is, as its function, provided so that any metals used in electrodes may not absorb the above light waves.
Thereafter, a metal thin film is vacuum-deposited on the surface of the buffer layer, followed by patterning to form electrodes denoted by reference numerals 14a and 14b in FIG. 6. In the electrode patterning, photolithography may be used like the case of the formation of optical waveguides as described above. More specifically, usable are an etching process in which, e.g., Ti or Cr is uniformly vacuum-deposited to form a film, thereafter Au is vacuum-deposited thereon, and then unnecessary portions are removed by chemical etching; and a lift-off process in which a photoresist is applied in a pattern, thereafter a metallic material is vacuum-deposited, and subsequently the photoresist is dissolved and removed.
The Ti or Cr is used to form a first layer of each electrode, for the reason that it also has a superior adherence to the substrate material such as the lithium niobate substrate crystal and to the SiO2 used as the buffer layer material. Also, the Au is used to form a second layer on the first layer, for the reason that the Au has superior electrical conductivity and environmental resistance and further makes it easy to carry out wire bonding.
Now, as stated above, the buffer layer functions so that the metals used in electrodes may not absorb the light waves propagated through the interior of optical waveguides, where there is a problem that may arise because of the presence of the buffer layer. For example, it is DC drift.
Here is explained what phenomenon the DC drift is. The electric field E of an optical waveguide region is expressed by the following mathematical expression (3), where V is the external applied voltage and g is the electrode gap.E=V/g  (3).
Note, however, that g is herein not the actual electrode gap G used in the mathematical expression (1), and an electrode gap value g is introduced which is actually effective in intuitionally understanding the matter.
According to the mathematical expression (3), E is kept constant as long as V is kept constant, and the optical power is kept at a constant value. However, although the external applied voltage V is kept constant, E changes with time, so that the optical power is not kept at a constant value. Such a phenomenon may come about. This is the DC drift. This DC drift occurs as an effect brought by actually effective capacitance component and resistance component in the buffer layer itself and at the interface between the buffer layer and the optical waveguide substrate. The DC drift must be kept from occurring, in order that waveguide type optical control elements such as photonic switches and light intensity modulators are actually put into use.
Where the buffer layer is removed in order to eliminate the DC drift, any measure substituting for the buffer layer must be taken in order to prevent the light waves from being absorbed by the electrodes. In particular, in single-mode optical fiber systems, the polarization does not stand constant with time. Hence, in waveguide type optical control elements used in such systems, it is required that the element shows the same behavior in respect to both the modes of the TE mode (transverse electromagnetic mode), in which the direction of vibration of the electric-field is parallel to the substrate surface and the absorption at the electrodes is not remarkable, and the TM mode (transverse magnetic mode), in which the direction of vibration of the electric-field is vertical with time and the absorption at the electrodes is remarkable, i.e., that the element is free of dependence on polarized light.
In waveguide type optical control elements utilizing a thermo-optic effect, materials having a high transparency and an appropriately large temperature dependence of refractive index, such as quartz glass and polymeric materials, are used for substrates. Then, a strip of a metallic conductor such as Cr is set only on one arm of an interferometer, and an electric current is passed therethrough to generate Joule heat, where this heat is utilized to change the refractive index of only the one arm to consequently produce a phase difference between both arms. This phase difference brings a change in the amount of output light. This is quite the same as what is brought by the electro-optic effect.
The metallic conductor absorbs light waves like the above electrodes, and hence, where the buffer layer is not used, the metallic conductor must be separated at a certain distance from the optical waveguides. Where the buffer layer is used, the metallic conductor can be set closer to the optical waveguides, but the step of providing the buffer layer is required.
As a solution for such a complicated problem, a structure is proposed in which an ITO film (indium-tin oxide film; In2O3:Sn film), which is a conductive oxide transparent in the visible region, is used in the control electrode (see German Patent Publication No. DE3724634 A1). According to this, the ITO film as an electrode is formed on optical waveguides, and a protective-film layer is loaded thereon.
In general, when light comes incident on a substance, some part of the light is reflected, the remaining part is absorbed in the substance, and further some part thereof is transmitted therethrough. In2O3-base conductive oxide materials are n-type semiconductors, in which carrier electrons are present and their movement contributes to electrical conduction. Carrier electrons in such a conductive oxide film reflect and absorb light of the near-infrared region. The more the carrier electrons are present in the conductive oxide film, the larger the amount of reflection and absorption of the near-infrared light is (see “Techniques of Transparent Conductive Films”, pp. 55-57, Ohmusha, Ltd.; Edited by Japan Society for the Promotion of Science) and the larger coefficient of extinction the conductive oxide film has.
ITO films used widely at present have a low electrical resistance, but have a carrier electron concentration of 8×1020/cm3 or more and shows remarkable reflection and absorption in the near-infrared region of from 1.3 to 1.6 μm in wavelength. The use of such films as control electrodes in the state they are in contact with or proximity to optical waveguides brings about a remarkable loss of the near-infrared light of from 1.3 to 1.6 μm in wavelength that travels through the interiors of optical waveguides. Accordingly, as control electrodes for optical waveguides making use of the infrared light of from 1.3 to 1.6 μm in wavelength, they are required to have a low carrier electron concentration.
Meanwhile, the resistivity ρ (electrical conductivity 1/ρ) of a substance depends on the product of the carrier electron concentration n and the mobility μ of carrier electrons (1/ρ=enμ; e: elementary electric charge). In order to materialize electrode materials having a low carrier electron concentration and a high electrical conductivity, the mobility μ of carrier electrons must be large. The mobility of carrier electrons in ITO films is approximately from 10 to 35 cm2/Vsec. The mobility of carrier electrons in the n-type semiconductors indium oxide (In2O3) materials is chiefly governed by the scattering of ionized impurities or the scattering of neutral impurities. (In regard to the impurities, impurities contained in the state of ions are called the ionized impurities, and impurities contained in the state of neutrality as a result of the adsorption of surplus oxygen to surroundings are called the neutral impurities.)
When impurity elements added in order to make the carrier electron concentration higher are at a high level, carrier electrons are scattered and the mobility of carrier electrons lowers. It is possible to lower the carrier electron concentration of ITO films by introducing oxygen into the ITO films at a high level, but the mobility of carrier electrons in the ITO films further lowers because of an increase in neutral impurities which is due to the introduction of oxygen, resulting in a very low electrical conductivity.